1. Field of the Invention
This invention relates to a suspension control device for a vehicle, which provides a comfortable ride (drive feeling) and an excellent driving performance to a driver at any vehicle running speed from a low speed to a high speed.
2. Description of Related Art
There has been known a conventional technique in which the spring rate of a suspension for a vehicle is varied in accordance with a vehicle running speed (as disclosed in Japanese Laid-Open Patent Application No. 63-46911, for example). According to this technique, the spring rate is controlled to be reduced to a value lower than a normal value when the speed of the vehicle is increased and exceeds a predetermined value. In this technique, no consideration is paid to the effect of an force acting on the vehicle due to air flow which is caused during the vehicle running (this force is hereinafter referred to as aerodynamic force").
There has been also known another conventional technique in which the damping rate of a suspension of a vehicle is varied in accordance with a vehicle running speed (as disclosed in Japanese Laid-open Patent Application No. 60-71315, for example). According to this technique, the damping rate is adjusted to a value higher than a normal value at high speed, and to a value lower than the normal value at low speed. In this invention, no consideration is also paid to the effect of the aerodynamic force occurring during the vehicle running.
The effect of the aerodynamic force on the motion of a vehicle is intensified as the vehicle running speed is increased. Therefore, in order to improve drive feeling, responsivity and safety of the vehicle, this effect of the aerodynamic force must be introduced into a calculation of at least one of variation of the spring rate and variation of the damping rate.
The effect of the aerodynamic force on the motion of a vehicle has not been hitherto studied in detail. However, the inventors of this application have clarified the details of this aerodynamic effect through their experimental analysis based on the principle of aeroelasticity.